1. Field of the Invention
The invention is directed to a method and an apparatus for processing the output signals of an optical earth-horizon sensor of an orbiting earth satellite with respect to variously warm oppositely located earth rims (earth anomaly) for forming an off-course signal which is a measurement for an off-course angle of a line of sight of the earth-horizon sensor in relation to a satellite/earth central point connecting line, wherein the earth-horizon sensor includes a periodically reciprocated chopper disk located in the focal plane of an input lens, the chopper disk having a diameter corresponding to an image of the earth, and a detector collecting the periodically interrupted radiations entering said input lens, wherein the output signals of the detector are amplified and demodulated with the chopper frequency in order to form the off-course signal and corrected in the presence of an earth anomaly by means of a correction or anomaly signal.
2. Description of Related Art
For the attitude control of such, e.g. geostationary, satellites, control or off-course signals are required which report the incorrect position of the satellite with respect to a reference position. For this purpose, optical earth-horizon sensors, which can be classed with the zero-seeking sensors, are used, among others, for two satellite axes. Such an earth-horizon sensor operates in the infrared range and is based on the mechanical vibrator or chopper principle. The infrared radiation of the earth is collected by means of an objective lens, consisting of germanium, and falls on a circular vibrator or chopper disk in the focal plane of the lens. This chopper disk has a diameter corresponding to the image of the earth and is periodically reciprocated with a determined amplitude, the chopper amplitude, and a determined frequency, the chopper frequency. The light of the two oppositely located earth horizons alternatively released at the chopper frequency, which light enters through the input lens and is interrupted by the chopper disk, is directed onto a detctor, e.g. a pyroelectric detector, via a secondary lens consisting of a spherical mirror segment and a prism via a spectral filter for the infrared range.
The output signal of the detector is amplified and subsequently demodulated with the chopper frequency. If the line of sight of an earth-horizon sensor is directed exactly on the earth central point, i.e. if the satellite is located in the above-mentioned reference position, then the light energy received by the detector from the two earth-horizons is identical. In this case, the earth-horizon sensor supplies a zero signal by means of the demodulation of the output signal. However, if the line of sight of the earth-horizon sensor diverges from the satellite/earth central point connecting line, i.e. if the satellite is located in an incorrect position, then the detector receives different light energies from the two earth rims, the difference being a measurement of the off-course angle of the line of sight of the earth-horizon sensor in relation to the satellite/earth central point connecting line, i.e. it is a measurement for the incorrect position of the satellite.
In the previously described processing of the detector output signals, it is assumed that the two oppositely located earth rims have the same temperature, so that identical energy quantities are received by the detector in the reference position of the satellite and a zero signal is accordingly reported. However, this is a theoretical ideal case. Normally, the oppositely located earth horizons under consideration have different temperatures, which is commonly designated as an earth anomaly. In this case, the earth-horizon sensor would also report an ostensible off-course signal in the reference position of the satellite, which off-course signal is based solely on this earth anomaly. This ostensible off-course signal is, accordingly, in fact a zero point error of the earth-horizon sensor due to different radiations of the oppositely located earth horizons. This anomaly error is only slight, but in extreme cases can amount to approximately +20% of an angular degree corresponding to the measuring range of the earth-horizon sensor. Since geostationary satellites are used, among other things, for producing directional radio links and guided television links to the earth, this anomaly error must be corrected. A possibility for such an anomaly correction is described in the applicant' s copending application Ser. No. 745,020.
It can be shown that the anomaly portion of the off-course signal depends only on the earth anomaly and the chopper amplitude and--at least in small off-course angles --is even independent of the off-course signal. According to the above-mentioned suggestion, the earth-horizon sensor is operated with different chopper amplitudes on the basis of this knowledge. This can be effected, for example, intermittently or with the aid of a modulation of the chopper amplitude. The use of two earth-horizon sensors which are operated with different amplitudes is also possible. The respective off-course signals of the earth-horizon sensor are compared with values of the standard characteristic lines for chopper amplitudes without earth anomaly in a common off-course angle. If the measured values cannot be assigned to a common off-course angle then an earth anomaly is present, since the measured values and the value on the standard characteristic line could otherwise be made to overlap. The characteristic lines assigned to the measured values are subsequently displaced in such a way that these characteristic lines can be made to overlap with the standard characteristic line. The magnitude of this displacement corresponds to the anomaly error and is a direct measurement for the earth anomaly.
The anomaly correction noted above is based on the following principles. If there is no earth anomaly, the characteristic line of the earth-horizon sensor can be approximately shown by means of the following formula: ##EQU1##
The formula is applicable for the range ##EQU2##
In the formula:
U.sub.D.alpha. =the sensor off-course signal which is dependent on the the off-course angle .alpha. and is measured as electrical current; PA1 K=a constant proportionality factor which is determined by means of the sensor geometry; PA1 A=chopper amplitude; PA1 .alpha.=off-course angle of the line of sight of the earth-horizon sensor; PA1 K.sub.A =a mechanical, likewise constant transmission factor.
The entire characteristic line extends over an angle area of approximately .+-.18.degree.; the measuring range used for the sensor off-course angle is approximately .+-.1.degree.. It can be seen that the characteristic line can be linearized for small off-course angles, specifically by means of the following formula: EQU U.sub.D.alpha..sup.x =K.multidot.K.sub.A .multidot..alpha. (2)
If the two earth rims radiate different energies then the entire characteristic line U.sub.D is composed of the above uninterrupted characteristic line U.sub.D.alpha. and an anomaly portion U.sub.DA : EQU U.sub.D =U.sub.DA +U.sub.D.alpha. ( 3)
The anomaly portion U.sub.DA is then ##EQU3##
On the basis of the above formula 1, 3 and 4, then, there follows for the characteristic line U.sub.D of the earth-horizon sensor ##EQU4## or for small values .alpha. ##EQU5##
It follows from formula 4 to 6 that, as indicated above, the anomaly portion of the sensor off-course signal depends only on the anomaly V and the chopper amplitude A.
If the sensor off-course signal is now measured at two different chopper amplitudes A and A1, the anomaly portion U.sub.DA will be independent of the anomaly V at least for small angles. For small angles ##EQU6## applies for the anomaly portion and ##EQU7## applies for the characteristic line U.sub.D.alpha..sup.x which is simplified, corrected and true for small off-course angles .alpha..
For the earth anomaly V there follows ##EQU8## from which it follows that the earth anomaly depends only on the chopper amplitude and the sensor off-course signals at the different chopper amplitudes. Of course, for large off-course angles, the complete formula for the characteristic line must be made use of.
In this anomaly correction, the output signals of the detector are corrected with their noise portions affected by the amplification and signal processing.